Method for the thermal treatment of foundry pieces made from an alloy based on aluminium and foundry pieces with improved mechanical properties

ABSTRACT

A method of heat-treating a casting made of an aluminum-based alloy including an alloy of aluminum, silicon, and magnesium comprising heat treating the casting at a first temperature range for a first duration; gradually cooling the casting to a second temperature having a second temperature range; maintaining the casting at the second temperature range for a second duration; quenching the casting; and age hardening the casting.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is the National Stage of InternationalApplication No. PCT/FR03/00887, filed on Mar. 20, 2003, which claims thebenefit of French Application No. 02/03530, filed on Mar. 20, 2002.

BACKGROUND OF THE INVENTION

The present invention relates in general to heat treating casting alloysbased on aluminum and containing silicon, and also to the resultingcastings.

Aluminum-based casting alloys comprise various composition families,most of which are suitable for structural hardening by heat treatment.In particular, mention can be made of the aluminum/silicon/magnesiumfamily typically represented by AlSi7% Mg0.3%, AlSi7Mg0.6%, and AlSi10%Mg0.3% type alloys, and the aluminum/silicon/copper/magnesium familytypically represented by AlSi(5% to 10%)Cu(2% to 3.5%)Mg(0.2% to 0.3%)type alloys.

All those alloys are widely used for the mass production of automobilecomponents, for example cylinder heads that are subjected to very highstress while in use. In order to maximize the mechanical properties ofsuch alloys, at least in cases of the most severe stresses, it is usualto carry out heat treatment consisting of solution heat treatment andquenching, followed by age hardening for structural hardening.

One drawback of that kind of treatment is that it can make the alloyvery difficult to machine, particularly with structurally hardenedalloys including little or no copper (typically, at contents of not morethan 1%).

In particular, the machining of very fine threads (for example, threadsfor fastening injectors to diesel engine cylinder heads), long,small-diameter drilling and the deburring of machined surfaces canpresent problems (for example, burrs that cannot be broken up aredifficult to eliminate by brushing).

Document EP-A-1 065 292 describes a method of cooling a workpiece madeof light alloy following solution heat treatment of the workpiece, whichcan be thought of as staged quenching, by immersing the workpiece in abath of salt so as to bring its temperature rapidly to a value lying inthe range 350° C. to 450° C. Such a known method has the effect ofincreasing the high-temperature tensile strength of the material of theworkpiece, but does not in any way resolve the problems relating tomachinability. Moreover, that method results in weaker characteristicsat room temperature, which are unacceptable for applications of thecombustion engine cylinder head type.

Thus, to date, there is no technique for facilitating the machining ofparts made of an alloy of the type with structural hardening or thelike.

SUMMARY OF THE INVENTION

The present invention aims at mitigating those drawbacks and at makingit possible to obtain castings offering a good compromise between theintrinsic performance which is demanded of them, particularly in termsof their ability to withstand various kinds of stress, of theirmachinability, and of their suitability for deburring after machining.

To this end, in a first aspect, the invention provides a method ofheat-treating a casting made of an aluminum-based alloy, particularly analloy of aluminum, silicon, and magnesium, and optionally copper, themethod being characterized in that it comprises the following steps:

-   -   solution heat treating the casting at a first temperature range        for a first duration;    -   gradually cooling the casting to a second temperature lying        within a second temperature range lower than the first range;    -   continuing solution heat treatment of the part by maintaining it        at the second temperature range for a second duration;    -   quenching the part; and    -   age hardening the part.

A number of preferred, but non-limiting, aspects of the method are asfollows:

-   -   the second temperature range is selected in such a manner that        the treated alloy has its tensile strength reduced by an amount        in the range about 10% to about 40%, and preferably in the range        about 15% to about 35%, relative to the tensile strength which        would be obtained with a single solution heat treatment at the        first temperature range and for a duration equal to the sum of        the first and second durations;    -   the second temperature range is lower than the first temperature        range by about 8% to about 14%;    -   for an alloy having a low copper content (typically 1% by        weight, or less), the first temperature range is about 510° C.        to about 550° C., and preferably about 520° C. to about 540° C.;    -   the first duration then lies in the range about 1 hour (h) to        about 4 h, and preferably in the range about 1 h to about 2 h;    -   the second temperature range is then about 455° C. to about 485°        C., and preferably is about 460° C. to about 480° C.;    -   the second duration then lies in the range about 1 h to about 5        h, and preferably in the range about 1 h to about 3 h; and    -   the duration of step (b) then lies in the range about 30 minutes        (min) to about 3 h and 30 min, and preferably in the range about        1 h to about 2 h and 30 min.

In a second aspect, the present invention also provides a casting madeof an aluminum-based alloy, particularly, in an alloy of aluminum,silicon, and magnesium, and optionally copper, with, in particular, acopper content of less than about 1% by weight, and presenting improvedmachinability, the casting being characterized in that it presents:

-   -   a tensile strength lying in the range 220 mega pascals (MPa) to        300 MPa;    -   a 0.2% elastic limit lying in the range 170 MPa to 270 MPa;    -   a Brinell hardness number lying in the range 75 to 110.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, aims, and advantages of the present invention will becomemore apparent on reading the detailed description of a preferredimplementation thereof, given by way of a non-limiting example and withreference to the accompanying drawings, in which:

FIG. 1 shows the microstructure of a first type of alloy treated inaccordance with prior art;

FIG. 2 shows the microstructure of this same type of alloy treated inaccordance with the present invention;

FIG. 3 shows the microstructure of a second type of alloy treated inaccordance with the present invention;

FIG. 4 shows the microstructure of the same type of alloy treateddifferently from the present invention, and

FIG. 5 is a diagram of strength/elongation mechanical properties,showing the properties which can be obtained with the invention.

DETAILED DESCRIPTION

The invention is described in detail below.

Heat treatment based on the invention is carried out by putting intosolution at two temperature plateaus. The first plateau is implementedin a high temperature range, i.e. in the temperature range which isusual for solution heat treatment of the alloys in question, that theperson skilled in the art will define using well-known points ofreference. Typically, for alloys containing less than 1% copper byweight, the first temperature lies in the range about 510° C. to about550° C., preferably in the range about 520° C. to about 540° C., andmore particularly around 530° C. For alloys with a higher coppercontent, the temperature will be lower, for example in the range about475° C. to about 515° C., and, for alloys with a copper content of 2% to3% by weight, preferably around 495° C. Mainly for economic reasons, thefirst plateau is limited to durations in the order of 1 h to 4 h, andpreferably 1 h to 2 h, in the knowledge that extending this plateau doesnot lead to any significant improvement in the final properties of thematerial.

The first plateau is followed by a second plateau in a secondtemperature range that is lower. Still for an alloy containing not morethan 1% copper by weight, the temperature range is about 455° C. toabout 485° C., preferably about 460° C. to about 480° C., and morepreferably around 465° C. (it is specified that for an alloy with acopper content of 2% to 3% by weight, the second temperature range isadvantageously 425° C. to 455° C., and more preferably around 450° C.).

More generally, the second temperature range is lower than the firsttemperature range by about 8% to about 14%.

The duration of the second step is in the order of 1 h to 5 h,preferably 1 h to 3 h. Indeed, it appears that extending the retentiontime at the second plateau does not lead to any significant changes infinal properties, thus, once again, in economic terms, it is worthreducing the retention time.

Between the two solution heat treatment plateaus, cooling is effected insuch a manner as to move gradually from the highest temperature to thelowest temperature in a duration lying in the range 30 min to 3 h 30min. Preferably, again mainly for economic reasons, the duration lies inthe range 1 h to 2 h 30 min.

After the second, lower-temperature solution heat treatment plateau,quenching is applied under the usual conditions, such as quenching inwater.

Lastly, age hardening is performed in order to develop hardeningprecipitation of the alloy. This age hardening operation is selectedfrom amongst the usual temperature ranges and durations; depending onthe properties desired, there may be under-age hardening, age hardeningto peak tensile strength, or over-age hardening.

The temperature of the second solution heat treatment plateau, whenchosen as determined above, makes it possible for the tensile strengthproperties of the alloy thus treated to be reduced by an amount lying inthe range about 10% to about 40%, and preferably in the range about 15%to about 35%, in comparison with the properties which would be obtainedwith a single solution heat treatment at the first temperature and for aduration equal to the sum of the durations at the two plateaus(including the cooling stage between the first plateau and the secondplateau), and maintaining the same quenching and age hardeningconditions.

In practice, compared with conventional single-plateau heat treatment,the invention seeks a better compromise between the tensile strength,elongation, and quality index properties. More particularly, it has beenobserved that it is preferable to reduce properties by an amount in therange 15% to 35% in order to optimize the strength/elongationcombination, whilst benefiting from improved machinability.

Moreover, it has also been found that two-plateau solution heattreatment reduces very significantly the residual stresses present inthe workpiece after the heat treatment has been completed. This may beof significant advantage to the strength of parts that are subjected tomajor stress, particularly combustion engine cylinder heads.

The following examples will show how the invention works:

Comparison Between the Invention and the Prior Art in Terms of Impact onMicrostructure and on the Strength/Elongation Compromise

A cylinder head made of second melt AlSi7% Mg0.4% alloy, cast at lowpressure, weighing about 18 kilograms (kg), and subjected to prior artheat treatment (solution heat treatment at the maximum temperature,quenching, and age hardening) presents difficulties for machining:large-sized pieces of drilling swarf tend to wind themselves around thecutting tools (or else remain in the oil circuits, for example). It isdifficult, therefore, to remove them. Such machining problems areconnected with the alloy characteristics that are obtained after theknown heat treatment.

More precisely, after traditional solution heat treatment at 530° C. for5 h, followed by quenching in water at 90° C., and by age hardening at200° C. for 5 h, the mechanical characteristics obtained on the cylinderhead on its rocker-arm face were as follows:

Breaking strength 341 MPa Elastic limit at 0.2% deformation 298 MPaPlastic elongation 2.17% Brinell hardness number 112 Quality index 391MPa

-   -   Typical microstructure after the usual heat treatment reveals:    -   silicon spheroidized by remaining at a high temperature, as        shown in FIG. 1 of the drawings; and    -   optimum putting into solution, i.e. no Mg₂Si component was        observed that was not in the solution.

Instead of the known heat treatment, a heat treatment was performed onan identical cylinder head at a first temperature plateau of 530° C.,for 2 h, then at a second plateau of 465° C., for 2 h, leaving a 1 hperiod for the temperature to go from the first temperature to thesecond, then quenching in water at 90° C., and age hardening at 200° C.for 5 h.

The mechanical characteristics of the material were then as follows:

Breaking strength 231 MPa (−32%) Elastic limit at 0.2% deformation 207MPa (−30%) Plastic elongation 4.64% (+114%) Brinell hardness number 90(−20%) Quality index 331 MPa (−15%)

With the treatment of the invention, breaking strength, elastic limit,and hardness were decreased by an amount in the range 20% to 32%. Thedecrease is to the benefit of a very considerable increase in theplastic elongation (i.e. breaking elongation +114%).

In terms of microstructure, the heat treatment of the present inventionreveals the presence of silicon spheroidized by the high temperaturesolution heat treatment, as shown in FIG. 2 of the drawings, whilstreducing the strength or hardness in comparison with conventionalsolution heat treatment.

2) Impact of the Temperature at the Second Plateau

In order to find an optimum temperature at the second plateau, solutionheat treatment was carried out using two plateaus, with a secondtemperature lower than in the above example, i.e. at 450° C. and at 400°C., respectively. In both cases, solution heat treatment at 530° C. wascarried out for 2 h, and then solution heat treatment was carried outfor 3 h at 450° C. or at 400° C., with a duration of 90 min or 120 minto reach the second temperature.

Again, solution heat treatment was followed by quenching in water at 90°C. and age hardening for 5 h at 200° C.

But such second plateaus at lower temperatures were found to have causedtoo great a drop in the mechanical characteristics, as shown in thetable below:

Treatment in Two stages:

530° C. then, 450° C. or 400° C. Breaking strength (MPa)  207 (−39%) 169 (−50%) Elastic limit at 0.2%  151 (−49%) 8.41 (+288%) deformation(MPa) Plastic elongation (%) 4.22 (+94%) 8.41 (+288%) Brinell hardnessnumber   70 (−37%)   61 (−45%)

3) Impact of the First Step at High Temperature on Microstructure

Using a casting made of an alloy of the above-mentioned type, butmodified with strontium, a two-plateau solution heat treatment of theinvention was carried out, and also a single-plateau solution heattreatment at a temperature of 465° C. In the two-plateau case, the hightemperature plateau caused spheroidization of the silicon, as shown inFIG. 3, whereas in the single-plateau phase, spheroidization did notoccur, as shown in FIG. 4.

4) Impact of Two Plateaus on Mechanical Characteristics

The mechanical characteristics are also affected by the type of solutionheat treatment used, being improved by two-plateau solution heattreatment.

The diagram shown in FIG. 5 of the drawings shows the compromise betweenthe mechanical strengths (breaking strength Rm and elastic limit at 0.2%deformation Rp, in MPa) and breaking elongation (in %) aftersingle-plateau and two-plateau heat treatment, based on the temperatureat the single plateau and on the temperature at the second plateau. Thesquares (invention) and the triangles (single-plateau heat treatment)correspond to different temperatures, as shown in the diagram.

In FIG. 5, the continuous line T1 shows the variation in the breakingstrength/elongation pair as a function of the second temperature oftwo-plateau solution heat treatment, whilst the dashed line T2 shows thevariation of the same characteristics as a function of the temperatureof a single-plateau solution heat treatment. The dashed line T3 showsthe variation of the elastic limit/elongation pair as a function of thesecond temperature in two-plateau solution heat treatment, whereas thedashed line T4 shows the variation of the same characteristics as afunction of the temperature in single-plateau solution heat treatment.

In FIG. 5, and in association with the curves T1 and T3 obtained usingthe invention, there are shown the 10% and 40% decreases in strengthcorresponding to one range of the invention, and also the 15% and 35%decreases corresponding to a particularly preferred range of theinvention (hatching).

Moreover, by means of the positions of the curves T1 and T3 relative tothe curves T2 and T4, respectively, FIG. 5 shows that two-plateausolution heat treatment offers a strength/breaking elongation compromisethat is better than for the same material subjected to single-plateauheat treatment, and this applies whatever the temperature of the singleplateau.

5) Machinability Tests

In order to study the impact of the invention on machinability, twotests were carried out, one on a reference cylinder head made inaccordance with the prior art (single-plateau heat treatment at 530° C.for 5 h), as described in point 1) above, with the same alloy, and theother on a cylinder head made out of the same alloy and subjected toheat treatment at two temperature plateaus in accordance with theinvention, that is, at 530° C. for 2 h and at 465° C. for 2 h, withintermediate cooling for 1 h.

In both cases, solution heat treatment was followed by quenching inwater at 90° C. and then by age hardening at 190° C. for 5 h.

It was also found that the cylinder heads treated in accordance with theinvention were easier to machine. As an indication, easier machinabilitywas characterized by the fact that the average length of the pieces ofswarf was reduced and their average density was increased, leading toimproved fragmentability by the pieces of swarf, as shown in the tablebelow:

Average length of the pieces of swarf Average density Reference 2.6 cm0.11 g/cm³ According to 2 populations: (measured on an the invention 0.7cm* and 2.2 cm unsorted population) 0.64 g/cm³ *majority population

Of course, the present invention is not limited in any way to theembodiments shown and described above, and the person skilled in the artwill know how to effect numerous variations and modifications thereto.

In particular, the person skilled in the art will know how to vary theexact profile of temperature variation during the solution heattreatment, particularly with more than two plateaus, or even withplateaus within which the temperature can vary over a certain range.

Moreover, the person skilled in the art will know how to adapt thevarious parameters as a function mainly, but not exclusively, of:

-   -   the type of alloy used;    -   the weight and/or volume of the workpiece;    -   the intended application;    -   the type of machining to be carried out, etc.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A method of heat-treating a casting made of an aluminum-basedage-hardenable alloy comprising aluminum, silicon, and magnesiumcomprising the steps of: (a) solution heat treating the casting within afirst temperature range for a first duration; (b) gradually cooling thecasting to within a second temperature range; (c) continuing solutionheat treatment by maintaining the casting within the second temperaturerange for a duration from 1 hour to 5 hours; (d) quenching the casting;and (e) artificial age hardening the casting wherein said age hardenedcast alloy has a tensile strength which is reduced by an amount rangingfrom 10% to 40% relative to a tensile strength obtained with a singlesolution heat treatment.
 2. The method according to claim 1, wherein theamount is 15% to 35%.
 3. The method according to claim 1, wherein thesecond temperature range is lower than the first temperature range in anamount ranging from 8% to 14%.
 4. The method according to claim 1,wherein the first temperature range is from 510° C. to 550° C.
 5. Themethod according to claim 4, wherein the first temperature range is from520° C. to 540° C.
 6. The method according to claim 1, wherein the firstduration is from 1 hour to 4 hours.
 7. The method according to claim 6,wherein the first duration is from 1 hour to 2 hours.
 8. The methodaccording to claim 1, wherein the second temperature range is from 455°C. to 485° C.
 9. The method according to claim 8, wherein the secondtemperature range is from 460° C. to 480° C.
 10. The method according toclaim 1, wherein a second duration is from 1 hour to 3 hours.
 11. Themethod according to claim 1, wherein the duration of step (b) is greaterthan or equal to 30 minutes.
 12. The method according to claim 1,wherein the duration of step (b) is from 1 hour to two hours and thirtyminutes
 13. A method of heat-treating a casting made of analuminum-based age-hardenable alloy comprising aluminum, silicon, andmagnesium comprising the steps of: (a) solution heat treating thecasting within a first temperature range for a first duration; (b)gradually cooling the casting to within a second temperature range overa duration which is greater than or equal to 30 minutes; (c) continuingsolution heat treatment by maintaining the casting within the secondtemperature range for a second duration; (d) quenching the casting; and(e) artificial age hardening the casting wherein said age hardened castalloy has a tensile strength which is reduced by an amount ranging from10% to 40% relative to a tensile strength obtained with a singlesolution heat treatment.
 14. The method according to claim 13, whereinthe first temperature range is from 510° C. to 550° C.
 15. The methodaccording to claim 13, wherein the first duration is from 1 hour to 4hours.
 16. The method according to claim 13, wherein the secondtemperature range is from 455° C. to 485° C.
 17. The method according toclaim 13, wherein the second duration is from 1 hour to 5 hours.
 18. Themethod of claim 13, wherein the duration of step (b) is from 1 hour to 2hours and 30 minutes.
 19. A method of heat-treating a casting made of analuminum-based age-hardenable alloy comprising aluminum, silicon, andmagnesium comprising the steps of: (a) solution heat treating thecasting within a first temperature range for a first duration; (b)gradually cooling the casting to within a second temperature range overa duration which is greater than or equal to 30 minutes; (c) continuingsolution heat treatment by maintaining the casting within the secondtemperature range for a duration from 1 hour to 5 hours; (d) quenchingthe casting; and (e) artificial age hardening the casting wherein saidage hardened cast alloy has a tensile strength which is reduced by anamount ranging from 10% to 40% relative to a tensile strength obtainedwith a single solution heat treatment.